In the energization of three-phase motors or other apparatus with a plurality of individual phase conductors, different approaches have been devised in an effort to protect the load from damage if one of the individual phase conductors is interrupted, leaving only single-phase energy between the other two conductors. Many of these known arrangements are not oriented in a fail-safe direction. That is, when the fuse in one phase conductor blows or the power on that conductor is otherwise lost, this condition is detected and some resultant operation is initiated. It is therefore a principal object of the present invention to provide a protection system which produces a control signal which always remains above an effective reference or threshold level, indicating the presence of energy on all phase conductors of the mains. This control signal drops below the threshold value when a fault or open condition occurs, thus operating in a fail-safe manner.
Another important object of the invention is to provide an indication of trouble on a three-phase line when a low voltage or "brown-out" condition occurs.
Still another important object of the invention is to provide such an arrangement in a compact, inexpensive unit.